1. Field of the Invention
The present invention generally relates to implantable medical devices, and more particularly, relates to implantable cardiac pacing devices.
2. Description of the Prior Art
The medical treatment of bradycardia by artificial cardiac pacing has been known for some time. Currently, such treatment using an implantable pulse generator electrically coupled to the myocardial tissue via an insulated lead is well accepted.
Much has been done concerning efficiency of the therapy in relation to the acute and chronic condition of the pacing lead and electrode positioning. "Atrial Pacing: Efficacy and Safety" by Kerr, et al., Pace, Volume 12, July 1989, pages 1049-1054, describes a study of atrially paced patients. It is apparent from such studies that the ability to measure various lead parameters both acutely and chronically is most desirable.
Acute measurement of certain lead parameters can be accomplished using a pacing system analyzer. Typically this is an external device which is used at the time of implant to verify proper lead position and function. U.S. Pat. No. 4,141,367 issued to Ferreira describes such a pacing system analyzer which measures lead impedance as computed by measuring output capacitor voltage before and after delivery of a pacing pulse. U.S. Pat. No. 4,364,396 issued to Barthel teaches a pacing system analyzer which measures output energy by sensing output capacitor voltage before and after pacing.
Lead impedance and pulse output measurements are also made chronically by some implantable pulse generators. Normally this data is subsequently telemetered external to the patient for display and analysis. U.S. Pat. No. 4,332,256 issued to Brownlee et al., discusses an implantable pulse generator which uses change in voltage across the output capacitor to determine lead impedance. U.S. Pat. No. 4,899,750 issued to Ekwall uses a similar technique to prepare a moving average of the lead impedance value. European Patent Application No. 0 338 363 in the name of Hafelfinger et al., suggests that lead impedance measurements should be made automatically upon the occurrence of various events.
U.S. Pat. No. 4,337,776 issued to Daly et al., proposes to measure lead impedance by monitoring successive output pulses of different amplitude. U.S. Pat. No. 4,949,720 issued to Thompson measures lead current directly by monitoring the current through FET devices in parallel with the main output path.